Accident detection circuit of a voltage-type self-excited power converter

ABSTRACT

The accident detection circuit of the present invention is such that in a voltage-type self-excited power converter composed of anode reactors 13 and 23 connected in series, respectively, to power semiconductor elements 11 and 21 of self turn off type connected in series, there are provided anode reactor voltage direction-detecting circuits 16 and 26 which detect, respectively, that the voltage in the direction generated in the anode reactors 13 and 23 exceeds a predetermined value when the current flowing in the direction of from the anode to the cathode of the each element 11, 21 increases, anode current direction-detecting circuits 17 and 27 which detect, respectively, that the current flows in the direction of from the anode to the cathode of the each element 11, 21, delay circuits 18 and 28 which delay, respectively, the output signal of each arm current direction-detecting circuit by a certain period of time, and AND circuits 19 and 29 which designate the output signal of the each anode reactor voltage direction-detecting circuit 16, 26 and the output signal of the each delay circuit 18, 28, respectively, as the input signal, and the output signal of each AND circuit 19, 29 is designated as the accident detection signal of the converter.

DESCRIPTION

1. Technical Field

The present invention relates to an accident detection circuit whichrapidly detects accidents of a converter due to the arm short-circuitand the like, in a voltage-type self-excited power converter used in aflicker-suppressing device which suppresses flickers of the alternatingcurrent-receiving voltage, a reactive power compensating network whichcompensates the reactive power, and an active filter which suppressesthe harmonic of the alternating current-receiving current.

2. Background Art

One example of conventional three-phase voltage-type self-excited powerconverter comprises 6 arm circuits, and each arm circuit is composed ofa power semiconductor element of a self turn off type, a diode connectedin anti-parallel to this semiconductor element, and an anode reactorconnected in series to the anode of the semiconductor element and forsuppressing the current rising rate.

Now, in the voltage-type self-excited power converter having such astructure, when the semiconductor element of the arm circuit on the lowvoltage side is continued, if the semiconductor element of the armcircuit on the high voltage side is broken or continued due to somemisconduct, direct-current short-circuit accident current will flow fromthe direct-current circuit. Similarly, when both ends of the arm circuiton the high voltage side are short-circuited due to some accident suchas flashover, or when short-circuit accidents are caused between lineson the alternating-current side of the converter, similarly thedirect-current short-circuit current will flow to the semiconductorelement.

Conventionally, in order to detect such accidents and protect thesemiconductor element, there have been provided a current detector andan overcurrent relay for every arm circuit. These are to detect thecurrent flowing to the arm circuit by the current detector, to detectwhen this detected current becomes the overcurrent exceeding a certainvalue with the overcurrent relay, and to interrupt the short-circuitaccident current by giving an off-gate signal simultaneously to allsemiconductor elements which constitute the power converter, to protectthe power converter.

In said protecting method, there are disadvantages described below. Thatis, if it is tried to interrupt the current higher than theinterruptible current value where there is a current value which can beinterrupted as a rating of the semiconductor element by giving theoff-gate signal, the semiconductor element will be broken.

Furthermore, in order to output the off signal for interrupting theaccident current to the semiconductor element, some periods of time isrequired, since there is a delay time in the circuit which outputs theoff-gate signal.

Therefore, in such accidents, it is required to output the off-gatesignal before the accident current reaches the interruptible currentvalue of the semiconductor element after the detection of the accident.Therefore, conventionally, considering the time delay for outputting theoff-gate signal after detecting the overcurrent, the anode reactor hasbeen made large to make the rising of the accident current slow. As aresult, it leads to the increase of the circuit loss for dealing withthe energy of the anode reactor at the time of normal commutation,thereby the efficiency of the power converter decreases.

Or, there is a defect that, in order to interrupt the accident current,semiconductor elements having an interrupting current rating of largercapacity than the interrupting current rating required for interruptingthe current flowing at the normal operation must be used.

The object of the present invention is for removing the afore-mentioneddefects, and to provide an accident detection circuit of a voltage-typeself-excited power converter which can rapidly detect the accident inthe self-excited power converter before the short-circuit accidentcurrent reaches the overcurrent, and detect the accident in the powerconverter without making the anode reactor or the power semiconductorelement of self turn off type larger than being required.

DISCLOSURE OF INVENTION

The present invention includes voltage direction-detecting means, armcurrent direction-detecting means, and an output means. The voltagedirection-detecting means detects that the voltage in the directiongenerated in the anode reactor when the current flowing in the directionof from the anode to the cathode of the power semiconductor element ofself turn off type increases exceeds a predetermined value. The armcurrent direction-detecting means detects that the current flows in thedirection from the anode to the cathode of the power semiconductorelement of self turn off type. And, the output means delays the armcurrent direction-detecting signal by a certain period of time, and whenthis signal and the anode reactor voltage direction-detecting signalexist together, outputs the accident detection signal. Therefore, it candetect rapidly the accident in the power converter before the accidentcurrent reaches the overcurrent.

Furthermore, the present invention includes a voltagedirection-detecting means, a continuity period-control means, and anoutput means. The voltage direction-detecting means detects that thevoltage in the direction generated in the anode reactor when the currentflowing from the anode to the cathode of the power semiconductor elementof self turn off type increases exceeds a predetermined value. Thecontinuity period-control means determines the timing to initiate thecommutation action of the power semiconductor element of self turn offtype to control the continuity period. And, the output means delays thesignal which controls the continuity period by a certain period of time,and when this signal and the anode reactor voltage direction-detectingsignal exist together, outputs the accident detection signal. As aresult, it can detect the accident in the power converter, thereby thereis no need to provide a current detector to the power converter.

Furthermore, the present invention includes voltage direction-detectingmeans, voltage detection means and an output means. The voltagedirection-detecting means detects that the voltage in the directiongenerated in the anode reactor when the current flowing from the anodeto the cathode of the power semiconductor element of self turn off typeincreases exceeds a certain value. The voltage detection means detectsthe off-gate voltage between the gate and the cathode of the powersemiconductor element of self turn off type. The output means delays thereversed signal of the off-gate voltage-detecting signal by a certainperiod of time, and when this signal and the anode reactor voltagedirection-detecting signal exist together, outputs the accidentdetection signal. As a result, it can detect the accident in the powerconverter, and further it can be made to detect the breakage of thepower semiconductor element of self turn off type.

Furthermore, the present invention includes a voltagedirection-detecting means and a delay means. The voltagedirection-detecting means detects that the voltage in the directiongenerated in the anode reactor when the current flowing from the anodeto the cathode of the power semiconductor element of self turn off typeincreases exceeds a predetermined value. The delay means outputs theaccident detection signal, when the anode reactor voltage-detectingsignal detects that a certain period of time has passed. As a result,the accident in the power converter can be detected without addingparticularly a current detector or the like.

According to the accident detection circuit of the voltage-typeself-excited power converter, the accident in the voltage-typeself-excited power converter can be rapidly detected before theshort-circuit accident current reaches the overcurrent, thereby it isnot required to make the anode reactor, or the power semiconductorelement of self turn off type larger than being required.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a main circuit diagram showing one embodiment of thevoltage-type self-excited power converter of the present invention;

FIG. 2 is a structural view showing the first embodiment of the accidentdetection circuit of the voltage-type self-excited power converter ofthe present invention;

FIG. 3 is a detailed diagram of the anode reactor voltagedirection-detecting circuit of FIG. 2;

FIG. 4 is a detailed diagram of the arm current direction-detectingcircuit of FIG. 2;

FIG. 5 is a timing chart showing the continuity period-control signal,the on-gate signal and the off-gate signal for illustrating the actionof the first embodiment of the present invention;

FIG. 6 is a diagram illustrating the commutation action when the currentflows to the power semiconductor element of self turn off type of FIG.2;

FIG. 7 is a diagram illustrating the commutation action when the currentflows to the diode of FIG. 2;

FIG. 8 is a diagram illustrating the phenomenon at the time of accidentwhen the current flows to the power semiconductor element of self turnoff type of FIG. 2;

FIG. 9 is a diagram illustrating the phenomenon at the time of accidentwhen the current flows to the diode of FIG. 2;

FIG. 10 is a structural view showing the second embodiment of theaccident detection circuit of voltage-type self-excited power converterof the present invention;

FIG. 11 is a timing chart showing the continuity period-control signal,the on-gate signal and the off-gate signal for illustrating the actionof the second embodiment of the present invention;

FIG. 12 is a structural view showing the third embodiment of theaccident detection circuit of voltage-type self-excited power converterof the present invention; and

FIG. 13 is a structural view showing the fourth embodiment of theaccident detection circuit of voltage-type self-excited power converterof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will now be described in detail by way of examples withreference to the accompanying drawings. First, the voltage-typeself-excited power converter which is the object of the presentinvention will be described with reference to FIG. 1.

FIG. 1 shows a three-phase voltage-type self-excited power converter,which comprises arm circuits 1, 3 and 5 on the high voltage side, armcircuits 2, 4 and 6 on the low voltage side, and a direct currentcapacitor 7 to maintain the direct voltage constant, which is connectedto the direct current side of arm circuits 1 to 6.

The arm circuit 1 is composed of a power semiconductor element 11 ofself turn off type comprising, for example, GTO (gate turn-offthyristor), a diode 12 connected in anti-parallel to the semiconductorelement 11, and an anode reactor 13 connected in series to thesemiconductor element 11 and the diode 12, to suppress the currentrising rate di/dt of the power semiconductor element 11 of self turn offtype.

Similarly, the arm circuits 2 to 6 are composed of power semiconductorelements 21, 31, 41, 51 and 61 of self turn off type, diodes 22, 32, 42,52 and 62 connected in anti-parallel, respectively, to the semiconductorelements 21, 31, 41, 51 and 61, and anode reactors 23, 33, 43, 53 and63.

And to the connecting point of arm circuits 1 and 2 in the arm seriescircuit connecting in series arm circuits 1 and 2, to the connectingpoint of arm circuits 3 and 4 in the arm series circuit connecting inseries arm circuits 3 and 4, and to the connecting point of arm circuits5 and 6 in the arm series circuit connecting in series arm circuits 5and 6, there are connected, respectively, terminals R, S and T on thealternating current side, and to both terminals of the direct currentcapacitor 7, there are connected terminals P and N on the direct-current side.

In addition, in order to reduce the inclination dv/dt of voltage appliedto the power semiconductor elements 11 and 21 of self turn off type,generally a snubber circuit is arranged in parallel to the powersemiconductor elements 11 and 21 of self turn off type. And to deal withthe energy stored in the anode reactor at the time of commutation, areactor energy regenerative circuit or a circuit which connects inseries a resistance and a diode is connected in parallel to the anodereactors 13 and 23, but the description thereof is omitted since thesecircuits do not have the direct relationship with the present invention.

<First Embodiment>

FIG. 2 is a view showing the first embodiment of the present invention,and in the arm circuit 1 of FIG. 1, there are provided an anode reactorvoltage direction-detecting circuit (CKT) 16 which is one example of thevoltage direction-detecting means of the present invention, a currentdetector 14 which is one example of the arm current direction-detectingmeans of the present invention, an arm current direction-detectingcircuit 17, a delay circuit 18 which is one example of the delay meansof the present invention, and an AND circuit 19 which is one example ofthe output means of the present invention, and in the arm circuit 2,there are provided an anode reactor voltage direction-detecting circuit(CKT) 26 which is one example of the arm current direction-detectingmeans of the present invention, a current detector 24 which is oneexample of the arm current direction-detecting means of the presentinvention, an arm current direction-detecting circuit 27, a delaycircuit 28 which is one example of the delay means of the presentinvention, and an AND circuit 29 which is one example of the outputmeans of the present invention.

The anode reactor voltage direction-detecting circuit 16 is connected inparallel to the anode reactor 13, and detects that the voltage in thedirection generated in the anode reactor 13 exceeds a predeterminedvalue, when the current flowing in the direction of from the anode tothe cathode of the semiconductor element 11 increases. The anode reactorvoltage direction-detecting circuit 26 is connected in parallel to theanode reactor 23, and detects that the voltage in the directiongenerated in the anode reactor 23 exceeds a predetermined value, whenthe current flowing in the direction of from the anode to the cathode ofthe semiconductor element 21 increases.

The arm current direction-detecting circuit 17 inputs the currentdetected by the current detector 14, and detects that the current flowsin the direction of from the anode to the cathode of the semiconductorelement 11. The arm current direction-detecting circuit 27 inputs thecurrent detected by the current detector 24, and detects that thecurrent flows in the direction from the anode to the cathode of thesemiconductor element 21. The anode reactor voltage direction-detectingcircuits 16 and 26 are similar circuits with a forward voltage detectorwhich is applied to the semiconductor element for the electric powerconstituting a high-voltage separately excited converter, and arecomposed of a light-emitting element 61, a light-receiving element 62, aresistance 63, a light guide 64 and an amplifying circuit 65, as shownin FIG. 3.

The delay circuit 18 is to delay the output signal of the arm currentdirection-detecting circuit 17 by a certain period of time. And thedelay circuit 28 is to delay the output signal of the arm currentdirection-detecting circuit 27 by a certain period of time.

The AND circuit 19 inputs the output signal of the anode reactor voltagedirection-detecting circuit 16 and the output signal of the delaycircuit 18, and outputs an output signal, that is, an accident detectionsignal when both the output signals exist simultaneously. The ANDcircuit 29 inputs the output signal of the anode reactor voltagedirection-detecting circuit 26 and the output signal of the delaycircuit 28, and outputs an output signal, that is, an accident detectionsignal when both the output signals exist simultaneously.

The arm current direction-detecting circuits 17 and 27 are similarcircuits with the overcurrent relays 15 and 25 of FIG. 14, and either ofthem is composed of a light-emitting element 71, a light-receivingelement 72, a resistance 73, a light guide 74, an amplifying circuit 75,and a resistance 76, as shown in FIG. 4, and they are so constituted asto flow the current detected by the current detectors 14 and 24 to thelight-emitting element 71 and to detect it with the light-receivingelement 72.

The arm current direction-detecting circuits 17 and 27 may be composedof a level determination circuit which determines whether the currentdetected by the current detectors 14 and 24 of FIG. 2 is positive ornegative, differing from those shown in FIG. 4.

The action and effect of the first embodiment so constituted asdescribed above will now be described with reference to FIGS. 5 to 9.First, in order to help the understanding of the accident detectionmethod, the operation method of the voltage-type self-excited powerconverter and the phenomenon at the time of accident will be described,taking R-phase as an example.

The voltage-type self-excited power converter is operated by a knowntechnique such as the PWM control and the like. One example of themethod of PWM control and the phenomenon at the time of commutation isdescribed in page 108 and page 31 of "Semiconductor power convertingcircuit" published by the Electricity Society (the first edition: Mar.31, 1987), therefore detailed description will be omitted.

As shown in FIG. 5, the on-gate signal and the off-gate signal are givenalternately to the power semiconductor elements 11 and 21 of self turnoff type which constitute the arm circuits 1 and 2, so that the currentis not applied simultaneously to the arm circuit 1 on the high-voltageside and to the arm circuit 2 on the low voltage side to cause anydirect-current short-circuit accident. In addition, the control deviceof the self-excited power converter is operated under the signal whichdetermines the timing to determine the initiation of the commutationaction and controls the continuity period of the arm circuits 1 and 2 onthe low voltage side and the high voltage side.

Namely, for example, when the current is commutated from the arm circuit2 on the low voltage side to the arm circuit 1 on the high voltage side,the voltage-type self-excited power converter is operated so that theon-gate signal is given to the power semiconductor element 11 of selfturn off type which constitutes the arm circuit 1 after interrupting thepower semiconductor element 21 of self turn off type of the arm circuit2 by giving the off-gate signal thereto. As a result, the voltagewaveform on the alternating-current side of the power converter becomesa collected body of the voltage waveform of the rectangular wave whichdesignates the direct-current voltage as the peak value thereof, whichis a common knowledge.

As is shown in FIG. 6, at the time of normal commutation, if theoff-gate signal is given to the power semiconductor element 21 of selfturn off type when the current is flown to the power semiconductorelement 21 of self turn off type of the arm circuit 2 on the low voltageside, the current flowing to the power semiconductor element 21 of selfturn off type is interrupted. As a result, the current flowing to theanode reactor 23 on the low voltage side is also attenuated, thereby thevoltage having a polarity as shown in FIG. 6, that is, the voltage inthe direction generated when the current flowing from the anode to thecathode of the power semiconductor element 21 of self turn off typedecreases is generated in the anode reactor 23. Furthermore, as shown inFIG. 6 by a dotted line, since the current on the alternating-currentside flows to the arm circuit 1 on the high voltage side by interruptingthe power semiconductor element 21 of self turn off type, the voltagehaving the polarity as shown in FIG. 6 is generated transitionally inthe anode reactor 13 on the high voltage side.

As shown in FIG. 7, however, the commutation action is not performedeven if the off-gate signal is given to the power semiconductor element21 of self turn off type on the low voltage side, in order to performthe commutation action when the diode 22 of the arm circuit 2 on the lowvoltage side continues and the current flows via the diode 22. But, whenthe on-gate signal is given to the power semiconductor element 11 ofself turn off type on the high voltage side, the commutation action isstarted. As shown in FIG. 6 by a dotted line, during the commutationperiod when the current flowing from the diode 22 to thealternating-current side is changed to flow toward thealternating-current side via the power semiconductor element 11 of selfturn off type, the voltage in the direction generated when the currentflowing from the anode to the cathode of the power semiconductor element21 of self turn off type increases is applied to the anode reactor 23.Similarly, at this time, the voltage is applied also to the anodereactor 13 on the high voltage side, and the value thereof is roughlyhalf of the direct-current voltage.

Now, in order to simplify the description of the action of the firstembodiment, let's consider the case where the power semiconductorelement 11 of self turn off type of the arm circuit 1 on the highvoltage side is broken due to some misconduct, or both ends of the armcircuit 1 on the high voltage side is short-circuited due to flashoveror the like, when the power semiconductor element 21 of self turn offtype constituting the arm circuit 2 on the low voltage side is in thecontinuity period.

If the power semiconductor element 11 of self turn off type is broken orcontinued due to some misconduct when the current flows to the powersemiconductor element 21 of self turn off type on the low voltage side,the short-circuit current flows from the P side of the direct-currentcircuit to the N side of the direct-current circuit via circuits of theanode reactor 13, the power semiconductor element 11 of self turn offtype, the power semiconductor element 21 of self turn off type, and theanode reactor 23, as shown in FIG. 7. Therefore, the voltage of thedirect-current capacitor 7 is shared with the anode reactor 13 on thehigh voltage side and the anode reactor 23 on the low voltage sidesubstantially half-and-half, and the direction of the voltage applied tothe anode reactors 13 and 23 is the same as the voltage in the directionthat the current flowing from the anode to the cathode of the powersemiconductor elements 11 and 21 of self turn off type increases, thatis, the voltage having the polarity shown in FIG. 8.

If the power semiconductor element 11 of self turn off type is broken orcontinued due to some misconduct when the current flows to the diode 22on the low voltage side, the short-circuit current will flow, as shownin FIG. 9. The initial stage of the accident is similar to the normalcommutation as shown in FIG. 7, but since the power semiconductorelement 21 of self turn off type is supplied with the on-gate signalbecause of being in the continuity period, the short-circuit currentflows backward through the diode 22 on the low voltage side, and thenvia the power semiconductor element 21 of self turn off type on the lowvoltage side. It is completely the same as in FIG. 8 which illustratesthe case where the current flows to the power semiconductor element 21of self turn off type on the low voltage side, except that the currentflows in from the alternating-current side. To the anode reactor 23 onthe low voltage side, there is applied a voltage having the polarityshown in FIG. 9, that is, a voltage in the direction generated when thecurrent flowing in the direction of from the anode to the cathode of thepower semiconductor element 21 of self turn off type increases.

Also in the case where both ends of the arm circuit 1 on the highvoltage side are short-circuited due to flashover or the like, it isclear that a voltage corresponding to the direct voltage having apolarity shown in FIGS. 8 and 9 is generated in the anode reactor 23 onthe low voltage side. Furthermore, in the case where a short-circuitbetween lines is generated on the alternating-current side, if, forexample, a short-circuit between lines of the R phase and the S phase isgenerated while the arm circuit 2 on the low voltage side of the R phaseand the arm circuit 3 on the high voltage side of the S phase arecontinued, it is clear that the voltage in the direction generated whenthe current flowing from the anode to the cathode of the powersemiconductor element 31 of self turn off type of the arm circuit 3increases is applied to the anode reactor 33 provided in the arm circuit3 on the high voltage side of the S phase. It is a matter of course thatthe voltage in the similar direction is applied to the anode reactor 23on the low voltage side of the R phase.

As described above, the direction of the voltage applied to the anodereactor 23 on the low voltage side at the time of accident shown inFIGS. 8 and 9 is of the polarity in the reverse direction as that of thevoltage generated in the anode reactor 23 on the low voltage side whenthe current is commutated to the arm circuit 1 on the high voltage side,when the power semiconductor element 21 of self turn off type on the lowvoltage side shown in FIG. 5 is continued. Therefore, if a voltage inthe direction which increases the current flowing from the anode to thecathode of the power semiconductor element 21 of self turn off type isgenerated in the anode reactor 23, it shows the possibility that someaccident is caused in the power converter.

However, as the action to commutate the current to the arm circuit 2 onthe high voltage side from the state that the diode 22 is continued isdescribed in FIG. 7, when the current flows to the diode 22 on the lowvoltage side, a voltage in the direction generated when the currentflowing from the anode to the cathode of the power semiconductor element21 of self turn off type increases is applied to the anode reactor 23 onthe low voltage side, similarly as in the case of accident. However, atthe time of normal commutation, when a voltage is applied to the anodereactor 23 on the low voltage side in the direction which increases thecurrent flowing from the anode to the cathode of the power semiconductorelement 21 of self turn off type, the current flowing to the arm circuit2 is the one flowing via the diode 22, and the one flowing in thedirection of from the cathode to the anode of the power semiconductorelement 21 of self turn off type.

Therefore, when the current flowing to the arm circuit 2 on the lowvoltage side flows in the direction of from the anode to the cathode ofthe power semiconductor element 21 of self turn off type, and if avoltage in the direction generated in the anode reactor 23 on the lowvoltage side when the current flowing in the direction from the anode tothe cathode of the power semiconductor element 21 of self turn off typeincreases is applied to the anode reactor 23, it means that this is nota normal commutation action but an accident is caused.

Furthermore, in the above description, the description is made for thecase that the arm circuit 2 on the low voltage side is continued, forsimplifying the description, but it is clear that also in the case wherethe arm circuit 1 on the high voltage side is continued, an accident ofthe voltage-type self-excited power converter can be similarly detectedby replacing the low voltage side with the high voltage side, and thehigh voltage side with the low voltage side in the above description.

Therefore, if the high voltage side in the above description is replacedwith the non-continued side, it is clear that in the case of an accidentthat the arm circuit on the non-continued side is continued, an accidentthat both ends of the arm circuit 1 on the non-continued side isshort-circuited, or an accident of short-circuit between lines, and whenthe current flows from the anode to the cathode of the powersemiconductor element of self turn off type in the arm circuit 2, if itcan be detected that the voltage in the direction generated in the anodereactor when the current flowing from the anode to the cathode of thepower semiconductor element of self turn off type increases exceeds acertain value, it is possible to detect the occurrence of an accident athigh speed, before the accident current reaches, as in the conventionalcase, the overcurrent region.

Incidentally, the reason to detect the voltage higher than a certainvalue of the anode reactors 13 and 23 is that since the self-excitedpower converter lets the alternating current flow to the alternatingside, the voltage of the alternating current, that is, the voltagehaving a polarity in the both directions appears in the anode reactors13 and 23, with the change of the alternating current. Therefore, it isnecessary to detect the voltage higher than a certain value so as not todetect the voltage generated in the anode reactors 13 and 23 due to thecurrent change during the normal operation. However, the voltagegenerated in the anode reactors 13 and 23 due to the normal currentchange has quite small value compared to that of the direct voltage,because of a small inductance value, and the peak value of the waveformof the alternating voltage substantially equals to the value of thedirect voltage.

On the other hand, the voltage generated in the anode reactors 13 and 23at the time of accidents has a quite large value compared to the voltagegenerated by the normal current change, and as already described, it hasa voltage value of substantially half of the direct voltage or higher.Namely, the increase rate of the current at the time of accidents hasquite a large value than that generated normally.

Therefore, according to the first embodiment, anode reactor voltagedirection-detecting circuits 16 and 26 which detect the direction of thevoltage applied to the anode reactors 13 and 23 are provided in parallelto the anode reactors 13 and 23 and detect that the voltage in thedirection generated in the anode reactors 13 and 23 when the currentflowing in the direction of from the anode to the cathode of the powersemiconductor elements 11 and 21 of self turn off type increases exceedsa predetermined value. Namely, it is detected by the voltage generatedin the anode reactors 13 and 23 that when the current flows to the powersemiconductor elements 11 and 21 of self turn off type in the directionfrom the anode to the cathode, the current flowing to the powersemiconductor elements 11 and 21 of self turn off type in the directionfrom the anode to the cathode increases at the current increase ratehigher than that generated by a normal operation.

However, at the time of normal commutation, since the voltage having thesame polarity with the polarity at the time of accident is generated inthe anode reactors 13 and 23, current direction-detecting circuits 17and 27 which detect the direction of the current flowing to the armcircuits 1 and 2 are provided to the arm circuits 1 and 2, in order todetermine whether it is a phenomenon at the time of normal commutationor a phenomenon at the time of accident, and detect that the currentflows in the direction of from the anode to the cathode of the powersemiconductor elements 11 and 21 of self turn off type.

Delay circuits 18 and 28 which delay the output signal of the armcurrent direction-detecting circuits 17 and 27 by a certain period oftime are provided, and AND circuits 19 and 29 which designate the outputsignal of the anode reactor voltage direction-detecting circuits 16 and26 and the output signal of the delay circuits 18 and 28 as the inputsignal are provided, thus the accident of the converter can be detectedat high speed by distinguishing it from the phenomenon at the time ofnormal commutation, from the state that the anode reactorvoltage-detecting circuits 16 and 26 are operating, when certain periodof time has passed since the arm current direction-detecting circuits 17and 27 operated.

Now, the reason why the arm current direction-detecting signal isdelayed by a certain period of time will be described. In the same wayas the description of the commutation action shown in FIG. 7, the actionto commutate the current to the arm circuit 1 on the high voltage sidein the state that the current flows to the diode 22 on the low voltageside will be described. When the on-gate signal is given to the powersemiconductor element 11 of self turn off type on the high voltage side,in the state that the current flows to the diode 22 on the low voltageside, to commutate the current on the high voltage side, the currentwill flow to the power semiconductor element 11 of self turn off type inthe direction of from the anode to the cathode, and to the anode reactor13 on the high voltage side is applied a voltage in the directiongenerated when the current flowing from the anode to the cathode of thepower semiconductor element 11 of self turn off type increases.Therefore, if the commutation action toward the high voltage side isperformed when the diode 22 on the low voltage side is continued, such aproblem is caused that the accident detection circuit on the highvoltage side operates. In order to solve this problem, it is necessaryto delay the signal which detects the current flowing in the directionof from the anode to the cathode, during the time until the voltagegenerated in the anode reactors 13 and 23 disappear after the completionof commutation.

However, since there are provided circuits for detecting accidents bothin the arm circuits 1 and 2 on the high voltage side and the low voltageside, as is obvious from the above description, if an accident occurssimultaneously as the commutation action toward the high voltage side isperformed when the diode 22 on the low voltage side is continued, theaccident is detected after a period of time determined by the time delayof the delay circuit. However, since this time delay is essentiallycooperative with the normal commutation time of the current, theaccident current when the accident is detected is as large as thecurrent which performs the normal commutation action, and the conditionof the input signal of the AND circuit is satisfied before the accidentcurrent reaches the over current, thereby the accident can be detected.

The action of the first embodiment will now be described, assuming that,as in the above description, the arm circuit 1 on the high voltage sideis broken due to some misconduct, or is continued, or both ends of thearm circuit 1 on the high voltage side are short-circuited, when the armcircuit 2 on the low voltage side is continued. Here, the descriptionwill be made assuming that the arm circuit 1 on the high voltage side isbroken due to some misconduct, or is continued, or both ends of the armcircuit 1 on the high voltage side are short-circuited, when the armcircuit 2 on the low voltage side is continued.

When an accident occurs, or when the on-gate signal is given to thepower semiconductor element 11 of self turn off type in the state thatthe current flows to the diode 22 on the low voltage side, the currentflowing in the direction of from the anode to the cathode of the powersemiconductor elements 11 and 21 of self turn off type increases, and avoltage having a polarity shown in FIG. 7, that is, in the directiongenerated when the current flowing in the direction of from the anode tothe cathode of the power semiconductor elements 11 and 21 of self turnoff type increases is applied to the anode reactors 13 and 23.

As a result, the current flows to the light-receiving element 61 whichconstitutes the anode reactor voltage direction-detecting circuits 16and 26 shown in FIG. 3 to emit the light. The emitted light is guided tothe light-receiving element 62 by the light guide 64, and by thereciprocal action with the amplifying circuit 65, it can be detectedthat a voltage is applied to the anode reactor. At this time, thevoltage value which can be detected by selecting properly the resistancevalue connected in series to the light-emitting element 61 can be setoptionally. Accordingly, if the voltage detection value of the anodereactor voltage direction-detecting circuits 16 and 26 is set to acertain value which is not detected at the current increase rate duringthe normal operation, the anode reactor voltage direction-detectingcircuit 16 will operate when the on-gate signal is given to the powersemiconductor element 11 of self turn off type, when an accident occurs,or in the state that the current flows to the diode 22 on the lowvoltage side, and it will not operate during the normal operation.

In addition, when a voltage in which the current decreases in thedirection of from the anode to the cathode of the power semiconductorelements 11 and 21 of self turn off type is generated in the anodereactors 13 and 23, since the direction of the voltage is inverse to thedirection described above, the current does not flow to thelight-emitting element 61, therefore the anode reactor voltagedirection-detecting circuits 16 and 26 will not operate.

Next, if the on-gate signal at the time of commutation is given to thepower semiconductor element 11 of self turn off type on the high voltageside, the current will flow to the arm circuit 1 in the direction offrom the anode to the cathode of the power semiconductor element 11 ofself turn off type, thereby the current will flow to the light-emittingelement 71 which constitutes the arm current direction-detectingcircuits 17 and 27 shown in FIG. 4 to emit the light, and this lightwill be guided to the light-receiving element 72 by the light guide 74,and the electric signal obtained here will be amplified by theamplifying circuit 75. On the other hand, when the current flows to thearm circuits 1 and 2 in the direction of from the cathode to the anodeof the power semiconductor element 11 of self turn off type, the armcurrent direction-detecting circuits 17 and 27 will not operate, and thedetecting action will not be performed.

If it is a commutation action when the diode 22 on the low voltage sideis continued, the arm current direction-detecting circuit 17 on the highvoltage side will operate, but the signal is not input to the ANDcircuit 19 by means of the action of the delay circuit 18. Therefore, ifthe delay time of the delay circuit 18 is set so that there is caused atime delay corresponding to the commutation time at the normal current,any voltage is not generated in the anode reactor 13 after thiscommutation time has passed, therefore the anode reactor voltagedirection-detecting circuit 16 does not operate. Accordingly, since theinput condition of the AND circuit 19 is not satisfied, the accidentdetection circuit of the present embodiment will not operate.

In the case of accident that the arm circuit 1 on the high voltage sideis continued in the state that the current flows to the diode 22 on thelow voltage side, however, it is clear from the above description thatthe accident detection circuit on the high voltage side operates after acertain period of time has passed since the accident occurred.

Furthermore, it is also clear that the accident detection circuit on thelow voltage side operates after a certain period of time since thecurrent started to flow to the arm circuit 2 on the low voltage side inthe direction of from the anode to the cathode of the powersemiconductor element 21 of self turn off type.

Furthermore, when the current flows to the power semiconductor element21 of self turn off type, since the condition to be input from the armcurrent direction-detecting circuit 27, via the delay circuit 28, to theAND circuit 29 has already been realized in the place where the accidentoccurred, it is clear that the anode reactor voltage direction-detectingcircuit 26 operates at the time of occurrence of the accident, and thisdetection circuit operates immediately.

The above description assumes an accident that the arm circuit 1 on thehigh voltage side is continued, however, also in the case of accidentthat both ends of the arm circuit 1 on the high voltage side areshort-circuited, or in the case of accident between lines of R-phase andS-phase, the delay circuit 28 on the post stage of the arm currentdirection-detecting circuit 27 on the low voltage side has alreadyoperated, or operates after a certain period of time has passed since anaccident occurred, and the anode reactor voltage direction-detectingcircuit 26 operates immediately after the occurrence of accident,thereby the accident detection circuit of the present embodiment willoperate.

The above description applies similarly even if the low voltage side isreplaced with the high voltage side, and the high voltage side isreplaced with the low voltage side, therefore, the accident detectioncircuit of the present embodiment can rapidly detect the accident of thepower converter, regardless of accidents on the high voltage side or onthe low voltage side of the converter.

Furthermore, the above description has been made assuming that anaccident occurs that the arm circuit on the high voltage side iscontinued, but when it is an accident that both ends of the arm circuit1 on the high voltage side are short-circuited, the accident detectioncircuit on the high voltage side will not operate, but it is clear thatthe circuit on the low voltage side operates similarly to detect theaccident. Moreover, the accident is similarly detected when ashort-circuit accident between lines is caused.

Therefore, if an accident occurs, the anode reactor voltagedirection-detecting circuits 16 and 26 immediately operate, and if theaccident current flows to the arm circuit in the direction of from theanode to the cathode of the power semiconductor element of self turn offtype, the arm current direction-detecting circuits 17 and 27 willoperate, and the accident can be detected by the output signal to theAND circuits 19 and 29.

On the other hand, since the output signal does not appear to the ANDcircuits 19 and 29 by means of the delay circuits 18 and 28 at the timeof normal commutation, an accident of the power converter can be rapidlydetected by the accident detection circuit.

<Second Embodiment>

FIG. 10 shows the second embodiment of the present invention, and thesame parts with FIG. 2 are denoted with the same reference numerals andthe description thereof is omitted. The different point therewith, thatis, continuity period-control circuits 171 and 271, delay circuits 181and 281, and AND circuits 191 and 291 will be described here.

The continuity period-control circuit 171 controls the continuity periodof the power semiconductor element 11 of self turn off type on the highvoltage side, and the continuity period-control circuit 271 controls thecontinuity period of the power semiconductor element 21 of self turn offtype on the low voltage side. The delay circuit 181 delays the outputsignal of the continuity period-control circuit 171 by a predeterminedperiod of time, and the delay circuit 281 delays the output signal ofthe continuity period-control circuit 271 by a predetermined period oftime. The AND circuit 191 inputs the output signal of the delay circuit181 and the output signal of the anode reactor voltagedirection-detecting circuit 16, and outputs the accident detectionsignal when both output signals are simultaneously input. And the ANDcircuit 291 inputs the output signal of the delay circuit 281 and theoutput signal of the anode reactor voltage direction-detecting circuit26, and outputs the accident detection signal when both output signalsare simultaneously input.

As described above, the self-excited power converter determines thetiming to initiate the commutation action of the power semiconductorelements 11 and 21 of self turn off type, and is operated by a signalwhich controls the continuity period.

Furthermore, when the commutation action is performed similarly as atthe time of accident, the voltage in the direction generated in theanode reactors 13 and 23 when the current flowing in the direction offrom the anode to the cathode of the power semiconductor elements 11 and21 of self turn off type increases is applied to the anode reactors 13and 23 for the period of A immediately after the on-gate signal is givento the power semiconductor elements 11 and 21 of self turn off type,when the current flows to diodes 12 and 22, as shown in FIG. 11.

Therefore, as shown in FIG. 11, if it is detected from the output signalof the continuity period-control circuits 171 and 271 that it is thecontinuity period after a certain period of time has passed immediatelyafter the commutation of the power semiconductor elements 11 and 21 ofself turn off type, by using the delay circuits 181 and 281, the periodother than the phenomenon immediately after the commutation can bedetermined, thereby an accident can be detected by the signal detectedby the anode reactor voltage direction-detecting circuits 16 and 26 andthe logical multiplication. Namely, AND circuits 191 and 291 areprovided which designate the output signals of the anode reactor voltagedirection-detecting circuits 16 and 26, and the output signals of thedelay circuits 181 and 281 as the input signal. And, when a signalcontrolling the continuity period of the power semiconductor elements 11and 21 of self turn off type has passed for more than a certain periodof time, the anode reactor voltage direction-detecting circuits 16 and26 are operating, thereby an accident of the power converter can berapidly detected, distinguished from the phenomenon at the time ofnormal commutation.

According to the accident detection circuit of the second embodimentdescribed above, the following effects can be obtained in addition tothe effect of the afore-mentioned first embodiment. Since the current ofthe arm circuits 1 and 2 is not detected, as is the case of theembodiment of FIG. 2, it is not required to provide the currentdetectors 14 and 24 in the arm circuits 1 and 2.

<Third Embodiment>

FIG. 12 shows the third embodiment of the present invention, and thesame parts with FIG. 2 are denoted with the same reference numerals andthe description thereof is omitted. The different point therewith, thatis, voltage between G and K detection circuits 172 and 272, invertercircuits 173 and 273, delay circuits 182 and 282, and AND circuits 192and 292 will be described here.

The voltage between G and K detection circuit 172 detects the off-gatevoltage between G (gate) and K (cathode) of the power semiconductorelement 11 of self turn off type, and the voltage between G and Kdetection circuit 272 detects the off-gate voltage between G (gate) andK (cathode) of the power semiconductor element 21 of self turn off type.

The inverter circuit 173 inverts the output signal of the voltagebetween G and K detection circuit 172 and the inverter circuit 273inverts the output signal of the voltage between G and K detectioncircuit 272. The delay circuit 182 delays the output signal of theinverter circuit 173 by a predetermined period of time, and the delaycircuit 282 delays the output signal of the inverter circuit 273 by apredetermined period of time. The AND circuit 192 inputs the outputsignal of the delay circuit 182 and the output signal of the anodereactor voltage direction-detecting circuit 16, and outputs the accidentdetection signal when both output signals are simultaneously input. Andthe AND circuit 292 inputs the output signal of the delay circuit 282and the output signal of the anode reactor voltage direction-detectingcircuit 26, and outputs the accident detection signal when both outputsignals are simultaneously input.

According to the accident detection circuit of the third embodimentdescribed above, the following effects can be obtained in addition tothe effect of the aforementioned first embodiment. The anode reactorvoltage direction-detecting circuits 16 and 26 which detect thedirection of voltage applied to the anode reactors 13 and 23 areprovided in parallel to the anode reactors 13 and 23, to detect that thevoltage in the direction generated in the anode reactors 13 and 23 whenthe current flowing in the direction of from the anode to the cathode ofthe power semiconductor elements 11 and 21 of self turn off typeincreases exceeds a certain value. However, even at the time of normalcommutation, a voltage having the same polarity as the polarity at thetime of accident is generated in the anode reactors 13 and 23, thereforeit is necessary to determine whether it is a phenomenon at the time ofnormal commutation or it is a phenomenon at the time of accident.

Therefore, in the embodiment of FIG. 12, voltage between G and Kdetection circuits 172 and 272 for detecting the off-gate voltagebetween gate and cathode of the power semiconductor elements 11 and 21of self turn off type are provided, thereby the continuity state of thepower semiconductor elements 11 and 21 of self turn off type can bedetected, including the breakage of the semiconductor elements 11 and21.

Therefore, the output signals of the voltage between G and K detectioncircuits 172 and 272 are inverted via the inverter circuits 173 and 273,respectively, and these inverted signals are delayed by a certain periodof time by the delay circuits 182 and 282 to determine that the powersemiconductor elements 111 and 21 of self turn off type are in thecontinuity period when a certain period of time has passed immediatelyafter the commutation. And the output signals of the anode reactorvoltage direction-detecting circuits 16 and 26, and the output signalsof the delay circuits 182 and 282 are input to the AND circuits 192 and292. Therefore, an accident of the power converter can be rapidlydetected from the state that the anode reactor voltagedirection-detecting circuits 16 and 26 are operating when the powersemiconductor elements 11 and 21 of self turn off type are continued,and the breakage of the power semiconductor elements 11 and 21 of selfturn off type can be also detected.

<Fourth Embodiment>

FIG. 13 shows the fourth embodiment of the present invention, and thesame parts with FIG. 2 are denoted with the same reference numerals andthe description thereof is omitted. The different point therewith willbe described here. Namely, a delay circuit 183 is provided on the outputside of the anode reactor voltage direction-detecting circuit 16, theoutput signal of the anode reactor voltage direction-detecting circuit16 is delayed by a certain period of time, and the output signalobtained from the delay circuit 183 is dealt with as the accidentdetection signal. Furthermore, a delay circuit 283 is provided on theoutput side of the anode reactor voltage direction-detecting circuit 26,the output signal of the anode reactor voltage direction-detectingcircuit 26 is delayed by a certain period of time, and the output signalobtained from the delay circuit 283 is dealt with as the accidentdetection signal.

According to the fourth embodiment thus constituted, the action andeffects described below can be obtained. It is as described above thatthe voltage applied to the anode reactors 13 and 23 has a quite smallvalue compared to the voltage at the time of accident, in the state thatthe transitional phenomenon at the time of commutation has completed andthe current at the normal operation flows to the anode reactors 13 and23. Namely, after the time required for the normal commutation of thecurrent has passed, almost no voltage is generated in the anode reactors13 and 23. Therefore, since the delay circuits 193 and 293 which delaythe anode reactor detecting signal by a certain period of time areprovided, accidents of the power converter can be detected,distinguished from the phenomenon at the time of normal commutation,from the state that the anode reactor voltage direction-detectingcircuits 16 and 26 has been operating for more than a certain period oftime.

In the embodiments described above, the three-phase circuit comprisingthe arm series circuit connecting in series arm circuits 1 and 2, thearm series circuit connecting in series arm circuits 3 and 4, and thearm series circuit connecting in series arm circuits 5 and 6 shown inFIG. 1 is mentioned as the voltage-type self-excited power converter.But the present invention is not limited thereto, and a single-phasecircuit comprising two arm series circuits or a circuit comprising morethan four arm series circuits may obtain the similar effects.Furthermore, the arm series circuit may have two or more of armcircuits.

We claim:
 1. An accident detection circuit of a voltage-typeself-excited power converter having an arm series circuit in which aplurality of arm circuits are connected in series, said each arm circuitcomprising a power semiconductor element of self turn off type and ananode reactor for suppressing the current rising rate which is connectedin series to the anode included in the semiconductor element, whichcomprises:voltage direction-detecting means which detects, respectively,that a voltage generated in the anode reactor of said each arm circuitand applied in a direction exceeds a predetermined value when thecurrent flowing in the direction of from the anode to the cathode ofevery semiconductor element of said each arm circuit increases, armcurrent direction-detecting means which detects, respectively, that thecurrent flows in the direction of from the anode to the cathode of everysemiconductor element of said each arm circuit, delay means whichdelays, respectively, an output signal of said currentdirection-detecting means of said each arm circuit by a certain periodof time, and output means which designates the output signals as theaccident detection signal of said power converter when the output signalof said voltage direction-detecting means and the output signal of saiddelay means are simultaneously present in every said arm circuit.
 2. Anaccident detection circuit of a voltage-type self-excited powerconverter having an arm series circuit in which a plurality of armcircuits are connected in series, said each arm circuit comprising apower semiconductor element of self turn off type, a diode connected inanti-parallel to said semiconductor element and an anode reactor forsuppressing the current rising rate which is connected in series to theanode included in the semiconductor element, which comprises:voltagedirection-detecting means which is connected, respectively, in parallelto the anode reactor of said each arm circuit, and detects that avoltage generated in said each anode reactor and applied in a directionexceeds a predetermined value when the current flowing in the directionof from the anode to the cathode of said each semiconductor elementincreases, arm current direction-detecting means which is provided inevery said arm circuit and detects that the current flows in thedirection of from the anode to the cathode of said each semiconductorelement, delay means which is provided for every said arm circuit anddelays an output signal of said arm current direction-detecting means bya certain period of time, and output means which is provided for everysaid arm circuit, and inputs the output signal of said eachvoltage-direction-detecting means and the output signal of said eachdelay means, and designates the output signals as the accident detectionsignal of said power converter when both output signals aresimultaneously present.
 3. An accident detection circuit of avoltage-type self-excited power converter according to claim 1 or 2, inwhich said arm series circuits are connected in parallel in pluralnumbers, and said voltage direction-detecting means, said arm currentdirection-detecting means and said output means are provided for everyarm circuit of one arm series circuit among said arm series circuits. 4.An accident detection circuit of a voltage-type self-excited powerconverter according to claim 1 or 2, wherein the arm currentdirection-detecting means comprises a light-emitting element which emitsthe light when the current flows, a first resistance which is connectedin series to said light-emitting element and sets the voltage, a secondresistance connected in parallel to said first resistance and saidlight-emitting element, a light-receiving element for receiving thelight from said light-emitting element and converting it to an electricsignal, a light guide which guides the light from said light-emittingelement to said light-receiving element, and an amplifying circuit whichamplifies the electric signal obtained from said light-receivingelement.
 5. An accident detection circuit of a voltage-type self-excitedpower converter according to claim 1 or 2, wherein said arm currentdirection-detecting means comprises a level determination circuit whichdetects the current flowing to the anode reactor and determines whethersaid detected current is positive or negative.
 6. An accident detectioncircuit of a voltage-type self-excited power converter having an armseries circuit in which a plurality of arm circuits are connected inseries, said each arm circuit comprising a power semiconductor elementof self turn off type and an anode reactor for suppressing the currentrising rate which is connected in series to the anode included in thesemiconductor element, which comprises:voltage direction-detecting meanswhich detects, respectively, that a voltage generated in the anodereactor of said each arm circuit and applied in a direction exceeds apredetermined value when the current flowing in the direction of fromthe anode to the cathode of every semiconductor element of said each armcircuit increases, continuity period-control means which controls,respectively, the continuity period of the semiconductor element of saideach arm circuit, delay means which delays, respectively, an outputsignal of said each continuity period-control means by a certain periodof time, and output means which designates the output signals as theaccident detection signal of said power converter when the output signalof said voltage direction-detecting means and the output signal of saiddelay means are simultaneously present in every said arm circuit.
 7. Anaccident detection circuit of a voltage-type self-excited powerconverter having an arm series circuit in which a plurality of armcircuits are connected in series, said each arm circuit comprising apower semiconductor element of self turn off type, a diode connected inanti-parallel to said semiconductor element and an anode reactor forsuppressing the current rising rate which is connected in series to theanode included in the semiconductor element, which comprises:voltagedirection-detecting means which is connected, respectively, in parallelto the anode reactor of said each arm circuit, and detects that avoltage generated in the anode reactor of said each arm circuit andapplied in a direction exceeds a predetermined value when the currentflowing in the direction of from the anode to the cathode of said eachsemiconductor element increases, continuity period-control means whichis provided in said each arm circuit and controls the continuity periodof said semiconductor element, delay means which delays the outputsignal of said continuity period-control means by a certain period oftime, and output means which is provided for every said arm circuit, anddesignates the output signals as the accident detection signal of saidpower converter when the output signal of said voltagedirection-detecting means and the output signal of said delay means aresimultaneously present.
 8. An accident detection circuit of avoltage-type self-excited power converter according to claim 6 or 7, inwhich said arm series circuits are connected in plural numbers and inparallel, and said voltage direction-detecting means, said continuityperiod-control means, said delay means and said output means areprovided for every arm circuit of one arm series circuit among said armseries circuits.
 9. An accident detection circuit of a voltage-typeself-excited power converter having an arm series circuit in which aplurality of arm circuits are connected in series, said each arm circuitcomprising a power semiconductor element of self turn off type and ananode reactor for suppressing the current rising rate which is connectedin series to the anode included in the semiconductor element, whichcomprises:voltage direction-detecting means which detects, respectively,that a voltage generated in the anode reactor of said each arm circuitand applied in a direction exceeds a predetermined value when thecurrent flowing in the direction of from the anode to the cathode ofevery semiconductor element of said each arm circuit increases, voltagedetecting means which detects, respectively, the off-gate voltagebetween the gate and the cathode of every semiconductor element of saideach arm circuit, delay means which inverts and delays by a certainperiod of time, respectively, the output signal of said each voltagedetecting means, and output means which designates the output signals asthe accident detection signal of said power converter when the outputsignal of said voltage direction-detecting means and the output signalof said delay means are simultaneously present in every said armcircuit.
 10. An accident detection circuit of a voltage-typeself-excited power converter having an arm series circuit in which aplurality of arm circuits are connected in series, said each arm circuitcomprising a power semiconductor element of self turn off type, a diodeconnected in anti-parallel to said semiconductor element and an anodereactor for suppressing the current rising rate which is connected inseries to the anode included in the semiconductor element, whichcomprises:voltage direction-detecting means which is connected,respectively, in parallel to the anode reactor of said each arm circuit,and detects that a voltage generated in the anode reactor of said eacharm circuit exceeds a certain value when the current flowing in thedirection of from the anode to the cathode of said each semiconductorelement increases, voltage detecting means which is provided for everysaid arm circuit and detects, respectively, the off-gate voltage betweenthe gate and the cathode of every semiconductor element of said each armcircuit, delay means which is provided for every said arm circuit, andinverts and delays by a certain period of time, respectively, the outputsignal of said voltage detecting means, and output means whichdesignates the output signals as the accident detection signal of saidpower converter when the output signal of said voltagedirection-detecting means and the output signal of said delay means aresimultaneously present in every said arm circuit.
 11. An accidentdetection circuit of a voltage-type self-excited power converteraccording to claim 9 or 10, in which said arm series circuits areconnected in parallel in plural numbers, and said voltagedirection-detecting means, said voltage detecting means, said delaymeans and said output means are provided for every arm circuit of onearm series circuit among said arm series circuits.
 12. An accidentdetection circuit of a voltage-type self-excited power converter havingan arm series circuit in which a plurality of arm circuits are connectedin series, said each arm circuit comprising a power semiconductorelement of self turn off type and an anode reactor for suppressing thecurrent rising rate which is connected in series to the anode includedin the semiconductor element, which comprises:voltagedirection-detecting means which detects, respectively, that a voltagegenerated in the anode reactor of said each arm circuit and applied in adirection exceeds a predetermined value when the current flowing in thedirection of from the anode to the cathode of every semiconductorelement of said each arm circuit increases, and delay means which delaysthe output signal of said voltage direction-detecting means,respectively, by a certain period of time, and designates the outputsignal as the accident detection signal of said power converter.
 13. Anaccident detection circuit of a voltage-type self-excited powerconverter having an arm series circuit in which a plurality of armcircuits are connected in series, said each arm circuit comprising apower semiconductor element of self turn off type, a diode connected inanti-parallel to said semiconductor element and an anode reactor forsuppressing the current rising rate which is connected in series to theanode included in the semiconductor element, which comprises:anodereactor voltage direction-detecting means which is connected,respectively, in parallel to the anode reactor of said each arm circuitand detects that the voltage in the direction generated in the anodereactor of said each arm circuit exceeds a certain value when thecurrent flowing in the direction of from the anode to the cathode ofsaid each semiconductor element of said each arm circuit increases, anddelay means which delays the output signal of said voltagedirection-detecting means, respectively, by a certain period of time,and designates the output signal as the accident detection signal ofsaid power converter.
 14. An accident detection circuit of avoltage-type self-excited power converter according to claim 12 or 13,in which a plurality of arm series circuits are connected in parallel,and said voltage direction-detecting means and said delay means areprovided for every arm circuit of one arm series circuit among said armseries circuits.
 15. An accident detection circuit of a voltage-typeself-excited power converter according to any one of claims 1, 2, 6, 7,9, 10, 12 or 13, wherein the voltage direction-detecting means comprisesa light-emitting element which emits the light when the current flows, aresistance which is connected in series to said light-emitting elementand sets the voltage, a light-receiving element for receiving the lightfrom said light-emitting element and converting it to an electricsignal, a light guide which guides the light from said light-emittingelement to said light-receiving element, and an amplifying circuit whichamplifies the electric signal obtained from said light-receivingelement.